1. Field of Use
This invention relates to mobile concrete batch plants for mixing cement and aggregate (including sand) in proper proportions at a job site and for depositing the mix into mix trucks.
2. Description of the Prior Art
Mobile concrete batch plants of various types are disclosed in the prior art. The typical plant comprises a large framework having ground wheels and a tow hitch to enable the plant to be transported to and from a job site. Discharge means are located on the framework in an elevated position to supply mix trucks driving therebeneath. A cement storage bin and an aggregate storage bin are mounted on the framework and are adapted to be loaded by suitable means such as front end loaders or the like. Means are provided to supply measured quantities or batches of cement and aggregate from the storage bins to the discharge means for deposit into the mix trucks.
U.S. Pat. No. 3,207,327, issued Sept. 21, 1965 to R. F. Long for "Concrete Batch Plant" shows a prior art plant wherein sand and aggregate are transported from their storage bins by separate belt conveyors to belt-type conveyor scales for weighing and for subsequent deposit into a discharge bin located therebelow. Cement is transported from a cement storage bin by an upwardly inclined screw conveyor to the top of a cement weigh bin wherein it is weighed prior to deposit into the discharge bin located therebelow along with the aggregate and sand. The mix in the discharge bin is then discharged onto the lower end of an upwardly inclined belt conveyor for final delivery to the elevated discharge means to supply the mix trucks.
U.S. Pat. No. 3,476,270, issued Nov. 4, 1969 to G. W. Cox et al. for "Mobile Concrete Batching Plant" shows another type of plant wherein an aggregate bin at the rear of the plant discharges into an aggregate weigh bin located therebelow and the latter discharges onto the lower end of an upwardly inclined belt conveyor which conveys the aggregate to the elevated discharge means at the front of the plant. The discharge means assume an elevated position when the front end of the plant is raised and supported in working position at the job site. Cement is transported from a cement storage bin by an upwardly inclined screw conveyor which discharges cement into the top of a cement weigh bin located above the elevated discharge means and wherein the cement is weighed prior to deposit into the discharge means along with the aggregate for delivery to the mix truck.
Each of the prior art plants requires a relatively large cement weigh bin (filled from the top and emptied from the bottom) wherein a batch of cement is weighed. In the Long patent the use of a cement weigh bin as shown necessitates a final upwardly inclined conveyor in order to deliver the mix to the mix truck, and this extends the overall length of the plant. In the Cox et al. patent the large cement weigh bin, being located above the elevated discharge means, requires extra headroom at the discharge end of the plant. Experience has also shown that other problems result from use of conventional cement weigh bins. For example, the discharge of cement from the discharge opening of a conventional cement weigh bin tends to be erratic and to lack continuity because the dry cement, although initially aerated to a loose and fluffy condition to encourage good flow characteristics, tends to lose its entrained air in the weigh bin and tends to arch above the weigh bin discharge opening. Use of additional air or vibration to loosen the cement, besides increasing the cost and complexity of batch plants, also results in irregular surges of cement through the weigh bin discharge opening and adds to the difficulty of controlling dry batch weighing of mix trucks.